java - Aligning buttons to center in BoxLayout -

i trying set buttons center using box.createhorizontalstrut() method. if use this.getwidth()/2 not work. how can center in frame.

code

package ch17; import java.awt.color; import java.awt.container; import java.awt.fontmetrics; import java.awt.graphics; import java.awt.gridlayout; import java.awt.event.actionevent; import java.awt.event.actionlistener; import javax.swing.*; import javax.swing.border.titledborder;  public class q17_1 extends jframe{     jbutton left = new jbutton("<=");     jbutton right = new jbutton("=>");     jpanel p1 = new jpanel();     jradiobutton rb1 = new jradiobutton("red");     jradiobutton rb2 = new jradiobutton("yellow");     jradiobutton rb3 = new jradiobutton("white");     jradiobutton rb4 = new jradiobutton("gray");     jradiobutton rb5 = new jradiobutton("green");     jpanel p2 = new jpanel();     message m = new message("welcome java");     public q17_1(){         setlayout(new gridlayout(3,1));         p1.setborder(new titledborder("select message panel background"));         buttongroup group = new buttongroup();         group.add(rb1);group.add(rb2);group.add(rb3);group.add(rb4);group.add(rb5);         rb1.setmnemonic('r');rb2.setmnemonic('y');rb3.setmnemonic('w');rb4.setmnemonic('g');         rb5.setmnemonic('n');         p1.setlayout(new gridlayout(1,5,5,5));         p1.add(rb1);p1.add(rb2);p1.add(rb3);p1.add(rb4);p1.add(rb5);         p2.setlayout(new boxlayout(p2,boxlayout.x_axis));         add(p1);         add(m);         p2.add(box.createhorizontalstrut(250));         p2.add(left);          p2.add(box.createhorizontalstrut(5));         p2.add(right);           add(p2);          left.addactionlistener((actionevent) -> {             m.moveleft();             repaint();         });          right.addactionlistener((actionevent)-> {             m.moveright();             repaint();         });          rb1.addactionlistener(m);         rb2.addactionlistener(m);         rb3.addactionlistener(m);         rb4.addactionlistener(m);         rb5.addactionlistener(m);     }      public static void main(string[] args) {         q17_1 frame = new q17_1();         frame.pack();         frame.setdefaultcloseoperation(jframe.exit_on_close);         frame.setlocationrelativeto(null);         frame.setvisible(true);     }   } 

i have tried, this.getwidth()/2, p2.getwidth()/2, etc. don't work , buttons still starting beginning of left side.

you use combination of borderlayout, flowlayout or gridbaglayout

for summary:

setlayout(new borderlayout()); //... p2.setlayout(new flowlayout()); add(p1, borderlayout.north); add(m); //... add(p2, borderlayout.south); 

the reason might consider using borderlayout of gridlayout core layout give remaining space component in center position. might not want, it's why i've used it.

both gridbaglayout , flowlayout layout it's containers around centre of container, gridbaglayout doing vertically , horizontally, flowlayout doing horizontally (by default)

import java.awt.borderlayout; import java.awt.flowlayout; import java.awt.gridlayout; import javax.swing.box; import javax.swing.buttongroup; import javax.swing.jbutton; import javax.swing.jframe; import javax.swing.jlabel; import javax.swing.jpanel; import javax.swing.jradiobutton; import javax.swing.border.titledborder;  public class q17_1 extends jframe {      jbutton left = new jbutton("<=");     jbutton right = new jbutton("=>");     jpanel p1 = new jpanel();     jradiobutton rb1 = new jradiobutton("red");     jradiobutton rb2 = new jradiobutton("yellow");     jradiobutton rb3 = new jradiobutton("white");     jradiobutton rb4 = new jradiobutton("gray");     jradiobutton rb5 = new jradiobutton("green");     jpanel p2 = new jpanel();     jlabel m = new jlabel("welcome java"); //  message m = new message("welcome java");      public q17_1() {         setlayout(new borderlayout());          p1.setborder(new titledborder("select message panel background"));         buttongroup group = new buttongroup();         group.add(rb1);         group.add(rb2);         group.add(rb3);         group.add(rb4);         group.add(rb5);         rb1.setmnemonic('r');         rb2.setmnemonic('y');         rb3.setmnemonic('w');         rb4.setmnemonic('g');         rb5.setmnemonic('n');          p1.setlayout(new gridlayout(1, 5, 5, 5));         p1.add(rb1);         p1.add(rb2);         p1.add(rb3);         p1.add(rb4);         p1.add(rb5);          p2.setlayout(new flowlayout());         add(p1, borderlayout.north);         add(m);         p2.add(left);         p2.add(right);          add(p2, borderlayout.south);          left.addactionlistener((actionevent) -> { //          m.moveleft(); //          repaint();         });          right.addactionlistener((actionevent) -> { //          m.moveright(); //          repaint();         });  //      rb1.addactionlistener(m); //      rb2.addactionlistener(m); //      rb3.addactionlistener(m); //      rb4.addactionlistener(m); //      rb5.addactionlistener(m);     }      public static void main(string[] args) {         q17_1 frame = new q17_1();         frame.pack();         frame.setdefaultcloseoperation(jframe.exit_on_close);         frame.setlocationrelativeto(null);         frame.setvisible(true);     }  } 

but if want use boxlayout practice p2. should put arguments in order center buttons???

because container's size dynamic, use horizontal glue instead

    p2.add(box.createhorizontalglue());     p2.add(left);     p2.add(right);     p2.add(box.createhorizontalglue());